The invention relates to starch used in the food industry, more specifically to starch used in processed foodstuff that, at least in one processing step, is subject to heat and or shear treatment.
In nature starch is available in an abundance surpassed only by cellulose as a naturally occurring organic compound. It is found in all forms of green leafed plants, located in their roots, stems, seeds or fruits. Starch serves the plant as food for energy during dormancy and germination. It serves similar purposes for man and animal as well as lower forms of life. Man, however, has found uses for starch that extend far beyond its original design as a source of biological energy. Practically every industry in existence uses starch or its derivatives in one form or another.
In foods and pharmaceuticals starch is used to influence or control such characteristics as texture, aesthetics, moisture, consistency and shelf stability. It can be used to bind or to disintegrate; to expand or to densify; to clarify or to opacify; to attract moisture or to inhibit moisture; to produce short texture or long (stringy) texture, smooth texture or pulpy texture, to produce a (semi)solid gel or a (viscous) fluid, soft coatings or crisp coatings. It can be used to emulsify or to form oil resistant films. Starch can be used to aid processing, packaging, lubrication or moisture equilibration. Starch truly serves as a multifunctional ingredient in the food industry.
A The most common sources of food starch are maize, potato, wheat tapioca, and rice. Maize is cultivated in warmer climates, with half of the world's production grown in the USA, its biggest crop. China, the second largest producer in the world, grows about 10%. Approximately 70% of the world's potato supply is grown in the cool, moist, climate of Europe and Russia. Wheat, requiring a more temperate climate, is primarily grown in the USSR, North America, and Europe. Approximately 90% of world rice production comes from South and South East Asia, while tapioca is cultivated in the narrow tropical band at about the equator.
The building blocks of carbohydrates such as starch are α and β-D glucose which contain six carbon atoms and form pyranose rings. Through enzymatic condensation, one molecule of water is split out between two molecules of glucose to form a bond. This condensation occurs predominantly between carbons 1 and 4 but occasionally between 1 and 6.
Where the α 1,4 linkage develops, a linear chained homopolymer results which we refer to as amylose. The length of this chain will vary with plant source but in general the average length will run between 500 and 2,000 glucose units. Traditionally, amylose is considered as being only linear in configuration but recent investigations indicate the presence of limited branching in some amylose molecules.
The second type of polymer in starch develops when the enzymatic condensation between glucose units occurs at carbons 1 and 6. This occasional linkage, along with the predominant 1,4 bonding, results in a branching effect and the development of a molecule much more massive in size than amylose but with linear chain lengths of only 25-30 glucose units. This molecule is called amylopectin.
All starches are made up of one or both of these molecules but the ratio of one to the other will vary with the starch source. Maize has about 25-28% amylose with the remainder being amylopectin. High amylose maize can run as high as 80%. Waxy maize has none and tapioca has about 17% amylose. Potato has about 17-25% amylose with the remainder being amylopectin.
As the plant produces the starch molecules, it deposits them in successive layers around a central hilum to form a tightly packed granule. Wherever possible, adjacent amylose molecules and outer branches of amylopectin associate through hydrogen bonding in a parallel-wise fashion to give radially orientated, crystalline bundles known as “micelles”. These micelles hold the granule together to permit swelling in (heated) water without the complete disruption and solubilisation of the individual starch molecules.
These highly orientated and crystalline micellular areas explain the ability of ungelatinised starch granules to rotate the plane of polarised light to produce characteristic interference crosses. This bi-refringent cross is one of the features used in identifying starch source. When the radial orientation of the crystalline micelle is disturbed, the bi-refringent cross disappears.
Gelatinisation temperatures are considered as ranges covering the temperatures at which loss of bi-refringence is first noticed and less than 10% remains. This temperature range is greatly influenced by the binding forces within the granule which vary with species. High amylose maize has much greater bonding force than the other maize varieties due to the high degree of linearity within the granule. On the other hand, ortho phosphate ester groups within the potato granule tend to weaken bonding and lower energy requirements to gelatinise.
When the starch granule is heated in water, the weaker hydrogen bonds in the amorphous areas are ruptured and the granule swells with progressive hydration. The more tightly bound micelles remain intact, holding the granule together. Bi-refringence is lost. As the granule continues to expand, more water is imbibed, clarity is improved, more space is occupied, movement is restricted and viscosity increased.
With the swelling of amylose-containing granules, some of the smaller amylose molecules are solubilised and leach out to re-associate into tight bundles which will precipitate if the starch concentration is low or will form a gel if the concentration is high. This is referred to as “set back” or retrogradation. The congealed paste will become cloudy and opaque with time and will eventually release water to shrink into a rubbery consistency.
Waxy maize has essentially no linear amylose molecules so its paste will remain flowable and clear. It will not gel or weep. Tapioca, having a small amount of amylose, gives a soft gel when pasted. Pastes from high amylose starch set to a very stiff gel.
To summarise the physical changes during gelatinisation: the granule swells and loses bi-refringence; clarity and viscosity increase; and smaller linear molecules dissolve and re-associate to form a gel.
In the unmodified form, starches have limited use in the food industry. Waxy maize starch is a good example. The unmodified granules hydrate with ease, swell rapidly, rupture, lose viscosity and produce weak bodied, very stringy and very cohesive pastes.